This invention relates to fabrication of bi-level circuits, and in particular to metallizing the surface and the via holes in the insulating substrate.
In a wide variety of circuit applications, it is desirable to form conductors on both major surfaces of an insulating substrate and interconnect the two sides through holes formed in the substrate. Such bi-level circuits may take the form of thick film circuits, thin film circuits or hybrid integrated circuits. In some instances, it is desirable to form the circuit on one surface of the substrate and provide a ground plane connection on the other surface. It is also possible to pattern circuits on both sides of the substrate, and further to provide thick film metallization on one surface and thin film metallization on the other surface.
One of the difficulties associated with forming bi-level circuits is coating the via holes in the substrate to provide the interconnections. Several methods have been proposed to accomplish such coating with varying degrees of success. One method is to plug the holes with a metal insert. One problem associated with this method is cracking of the substrate as a result of the high sintering temperatures needed for succeeding thick film layers if the coefficient of expansion of the metal is not well matched to that of the substrate. Another technique is to deposit a thick film paste over the hole and insure complete coverage of the walls by applying a vacuum and causing the paste to flow through the hole. In such processes, subsequent sintering can cause severe shrinkage of the paste resulting in the conductor breaking loose from the walls of the hole. Low shrinkage pastes, on the other hand, tend to have high viscosity and will not easily flow into the hole when the vacuum is applied. (See, for example, Kline et al "Thick Film Techniques for Printing Fine Lines and Plating Through Holes," 1973 International Microelectronic Symposium, pp. 3A-2-1 to 3A-2-6 (Oct. 22-24, 1973).) Another method for coating holes is drawing a deposited liquid metal through the hole with a vacuum nozzle (See, for example, U.S. Pat. No. 3,357,856). Handling molten metals can be difficult, and in general, drawing material through a hole by creating a vacuum could result in splattering. Several other methods for metallizing holes involving thin film technology, such as electroless deposition or evaporation and electroplating of metals on the side walls, have also been taught. (See, for example, U.S. Pat. No. 3,269,861). Such processes tend to require small lot size, and excessive care to insure adequate coverage without entrapment of air bubbles or various chemicals.
From an economical viewpoint, formation of surface conductors and interconnections by thick film techniques appears most desirable. However, it will be appreciated that prior art thick film processes in general have not produced reliable coverage of the via holes. This is due to the fact that the paste must have good flow properties, adhere to the substrate, and still be compatible with sintering needed for thick film processes. In addition, the resulting conductor must possess the desired conductivity after sintering. Furthermore, it is desirable that during processing the paste form a contact pad on the opposite surface for easy electrical contact to the circuit elements on that surface.
It is therefore a primary object of the invention to provide a method utilizing thick film pastes for forming surface conductors, coating via holes and establishing contact pads on the opposite surface of the substrate. It is a further object to coat the side walls of via holes without substantial cracks or voids and with the proper conductivity for interconnection of the two sides of the circuit. It is a further object of the invention to provide a composition of paste which is compatible with such a method.